Shale weathering profiles show Hg sequestration along a New York-Tennessee transect

Shale-derived soils have higher clay, organic matter, and secondary Fe oxide content than other bedrock types, all of which can sequester Hg. However, shales also can be Hg-rich due to their marine formation. The objectives of this study were to determine the concentration and phase partitioning of Hg in seven upland weathering profiles from New York to Tennessee USA and use geochemical normalization techniques to estimate the extent of Hg inheritance from weathering of shale bedrock or sequestration of atmospheric Hg. Total Hg concentrations in unweathered shale ranged from 3 to 94 ng/g. Total Hg concentrations decreased with depth in the Ultisols and Alfisols, with total Hg concentrations ranging from 18 to 265 ng/g. Across all shale soils and rocks, the oxidizable fraction of Hg (15% H₂O₂ extraction) comprised a large portion of the total Hg at 68% ± 8%. This fraction was dominated by organic matter as confirmed with positive correlations between Hg and %LOI, but could also be impacted by Hg sulfides. Across all sites, the reducible fraction of Hg (citrate-bicarbonate-dithionite extraction) was only 10% ± 4% of the total Hg on average. Thus, secondary Fe oxides did not contain a significant portion of Hg, as commonly observed in tropical soils. Although colder sites had a higher organic matter and sequestered more Hg, τ values for Hg indexed to Ti suggest that atmospheric deposition, such as pollution sources in Ohio River Valley, drove the highest enrichment of Hg along the transect. These results demonstrate that shale-derived soils have a net accumulation and retention of atmospheric Hg, primarily through stabilization by organic matter.